This invention relates to a disk substrate (may simply be referred to as a substrate hereinafter) for use in an information recording medium, such as a magnetic disk, an optical disk, and a magnetooptic disk and, in particular, to a glass substrate for an information recording medium as well as an information recording medium using the glass substrate.
As a substrate for an information recording medium, use is generally made of an aluminum substrate, an amorphous glass substrate (simply referred to as a glass substrate hereinafter), and a crystallized glass substrate. Among others, the glass substrate is gradually expanding its market share. This is because the glass substrate is easy to obtain desirable surface smoothness (i.e., a small surface roughness Rmax) and has strength sufficient to withstand a reduction in thickness and size, as compared with the other substrates.
Among various glass substrates, a chemically strengthened glass substrate with its surface chemically strengthened by ion exchange is widely known in the art. A typical chemically strengthened glass substrate is disclosed in Japanese Patent Publication (JP-B) No. 70262/92. The disclosed glass substrate comprises a glass substance including 60.0-70.0 wt % of SiO.sub.2, 0.5-14.0 wt % of Al.sub.2 O.sub.3, 10.0-32.0 wt % of alkali metal oxide, 1.0-15.0 wt % of ZnO, and 1.1-14.0 wt % of B.sub.2 O.sub.3 and having a value not smaller than a specific value for each of a linear expansion coefficient, pressure strength, and transverse rupture strength.
As a material for preparing the chemically strengthened glass substrate, two kinds of glass substances described in the following (a) and (b) are known.
(a) A glass substance including 55-62 wt % of SiO.sub.2, 10-18 wt % of Al.sub.2 O.sub.3, 2-10 wt % of ZrO.sub.2, 2-5 wt % of MgO, 0.1-3 wt % of BaO, 12-15 wt % of Na.sub.2 O, 2-5 wt % of K.sub.2 O, 0-7 wt % of P.sub.2 O.sub.3, and 0.5-5 wt % of TiO.sub.2, where the total amount of Al.sub.2 O.sub.3 and TiO.sub.2 is 13-20 wt % (see Japanese Unexamined Patent Publication (JP-A) No. 167245/89). PA1 (b) A glass substance including 64-70 wt % of SiO.sub.2, 14-20 wt % of Al.sub.2 O.sub.3, 4-6 wt % of Li.sub.2 O, 7-10 wt % of Na.sub.2 O, 0-4 wt % of MgO, and 0-1.5 wt % of ZrO.sub.2 (see Japanese Patent Publication (JP-B) No. 76224/94). PA1 (1) A substrate (will be referred to as a glass substrate I hereinafter) for an information recording medium, comprising a glass substance having a Young's modulus not smaller than 100 GPa and a liquidus temperature not higher than 1350.degree. C. PA1 (2) A substrate (will be referred to as a glass substrate II hereinafter) for an information recording medium, comprising a glass substance at least including TiO2 and CaO as glass components, the contents of which are selected so that a Young's modulus is not smaller than 100 GPa and a liquidus temperature is not higher than 1350.degree. C. PA1 (3) A substrate (will be referred to as a glass substrate III hereinafter) for an information recording medium, comprising a glass substance at least including TiO.sub.2 and CaO as glass components, the contents of which are selected so that a Young's modulus is not smaller than 100 GPa, a liquidus temperature is not higher than 1350.degree. C., and a viscosity is not smaller than 10 poises in a formable temperature range. PA1 (4) A substrate (will be referred to as a glass substrate IV hereinafter) for an information recording medium, comprising a glass substance at least including, as glass components, TiO.sub.2, CaO, MgO, and Al.sub.2 O.sub.3 along with at least Li.sub.2 O selected from a group consisting of Na.sub.2 O and Li.sub.2 O. The contents of the glass components are selected so that a Young's modulus is not smaller than 100 GPa, a liquidus temperature is not higher than 1350.degree. C., a viscosity is not smaller than 10 poises in a formable temperature range, and specific gravity is not greater than 3.5 g/cm.sup.3, PA1 (5) A substrate (will be referred to as a glass substrate V hereinafter) for an information recording medium, comprising a glass substance including as glass components 5-30 mol % of TiO.sub.2, 10-45 mol % of (MgO+CaO), where CaO is 1-45 mol %, 3-30 mol % of (Na.sub.2 O+Li.sub.2 O), 0-15 mol % of Al.sub.2 O.sub.3, and 35-60 mol % of SiO.sub.2. PA1 (6) A substrate (will be referred to as a glass substrate VI hereinafter) for an information recording medium, which is obtained by chemically strengthening one of the above-mentioned glass substrates I through V or a glass substance therefor.
As regards a hard disk apparatus, technical innovations are being made from time to time in order to keep the superiority to other recording/reproducing apparatuses using various media, such as an optical recording medium and a magnetooptic recording medium. One of the technical innovations is to rotate the magnetic disk at a high speed Such high speed rotation deserves one attempt to increase an access speed of a recording/reproducing magnetic head. The rotation speed is expected to exceed 10000 rpm hereafter, although it has been restricted to 5000-7000 rpm in the prior art.
As described above, in the magnetic disk, high speed rotation is desired. If the high speed rotation is sought by the use of the conventional glass substrate for an information recording medium, it is difficult to keep a flying height constant, i.e., a distance between the magnetic head and the magnetic disk at the time of recording and reproducing.